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Physical Review A

Optically induced spin precession and echo in an atomic beam

M. Zielonkowski, J. Steiger, U. Schünemann, Maarten DeKieviet, Rudolf Grimm

Published Nov 1, 1998• 25 citations
Publisher

Abstract

We report on spin precession experiments using a ${}^{7}\mathrm{Li}$ atomic beam, which offers a perturbation-free environment to investigate spin dynamics under the influence of external fields. The ground-state magnetic moments perform a Larmor precession in a ``fictitious magnetic field'' induced by an off-resonant, circularly polarized laser beam. For a fictitious magnetic field parallel to a real one, we observe additional rotation of the spins, whereas an echo is observed if both fields are perpendicular with respect to each other. The systematics of these phenomena are studied.

DOI
10.1103/physreva.58.3993

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A circularly polarized laser beam propagating through sodium vapor is shown to be deflected by the inhomogeneous magnetic field from a nearby electric current-carrying thin wire aligned parallel to the beam. The effect arises from the refractive index gradient induced by magnetic-field-modified optical pumping. The experimentally observed beam deflection is in good agreement with predictions from a $J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2\ensuremath{\leftrightarrow}J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ model for the full four-dimensional spatialtemporal evolution of the interaction between radiation field and atoms. The time of order 100 ns needed for the beam to switch between undeflected and deflected positions is determined by the average Larmor precession period and is 2 orders of magnitude shorter than the optical pumping time.
We report on a spin-echo phenomenon that is induced between Zeeman sublevels by two laser pulses close to an optical resonance. The echo formation is due to a Zeeman light shift during the second off-resonant pulse; this light pulse creates a fictitious magnetic field that leads to the phase reversal of the spins. Our experimental results on the 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​ sodium ground state are in good qualitative agreement with theoretical predictions.
We report experimental on-sky observations of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. A magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 and 100 km altitude. The experiment was performed at La Palma, and the uncertainty (0.2 kHz) in the measured Larmor frequency (≈260 kHz) corresponded to an error in the geomagnetic field of 0.3 mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere.